Strange metal phase of disordered magic-angle twisted bilayer graphene: from flatbands to weakly coupled Sachdev-Ye-Kitaev bundles

We use stochastic expansion and exact diagonalization to study the magic-angle twisted bilayer graphene (TBG) on a disordered substrate. We show that the substrate-induced strong Coulomb disorder in TBG with the chemical potential in the center of the flatbands drives the system to a network of weakly coupled Sachdev-Ye-Kitaev (SYK) bundles, stabilizing an emergent quantum chaotic strange metal (SM) phase of TBG that exhibits the absence of quasiparticles. The Gaussian orthogonal ensemble dominates TBG's long-time chaotic dynamics at strong disorder, whereas fast quantum scrambling appears in the short-time dynamics. In weak disorder, TBG exhibits exponentially decaying specific heat capacity and exponential decay in out-of-time-ordered correlators. The latter follows the Larkin-Ovchinnikov behavior of the correlator signaling the onset of the formation of a superconducting state. The result suggests the superconducting transition upon doping the system above the charge neutrality and weakening the disorder strength. We propose a finite-temperature phase diagram for Coulomb disordered TBG and discuss the experimental consequences of the emergent SM phase.Comment: 8 pages, 11 figure

Chiral spin liquid state of strongly interacting bosons with a moat dispersion: a Monte Carlo simulation

We consider a system of strongly interacting Bosons in two dimensions with moat band dispersion which supports an infinitely degenerate energy minimum along a closed contour in the Brillouin zone. The system has been theoretically predicted to stabilize a chiral spin liquid (CSL) ground state. In the thermodynamic limit and vanishing densities, $n\rightarrow 0$, chemical potential, $\mu$, was shown to scale with $n$ as $\mu\sim n^2\log ^2n$. Here we perform a Monte Carlo simulation to find the parametric window for particle density, $n \lesssim \frac{k^2_0}{82 \pi}$, where $k_0$ is the linear size of the moat (the radius for a circular moat), for which the scaling $\sim n^2\log ^2n$ in the equation of state of the CSL is preserved. We variationally show that the CSL state is favorable in a interval beyond the obtained scale and present a schematic phase diagram for the system. Our results offer some density estimates for observing the low-density behavior of CSL in time of flight experiments with a recently Floquet-engineered moat band system of ultracold atoms in Phys. Rev. Lett. 128, 213401 (2022), and for the recent experiments on emergent excitonic topological order in imbalanced electron-hole bilayers.Comment: 18 pages, 3 figures. Contribution to the Annals of Physics volume dedicated to the memory of Konstantin B. Efeto